The present invention relates to a screen printing apparatus for printing with high accuracy cream solder or paste used for forming a thick-film circuit on a substrate on which an electronic circuit is formed.
With the recent miniaturization of electronic appliances, normally, electronic components such as chip components are mounted on cream solder which has been printed on a substrate and then a reflowing oven is used to solder it to the cream solder.
The constructions of conventional screen printing apparatuses for printing paste such as cream solder on the substrate are described below with reference to FIGS. 3 through 5.
The construction of a first conventional screen printing apparatus is described below with reference to FIG. 3. A substrate 51 is positioned onto a stage 52. Then, a lower cylinder 53 moves the stage 52 upward so that the substrate 51 is positioned at a level immediately below a screen plate 55 fixed to a screen frame 54. Thereafter, a squeegee 56 is moved downward by a head cylinder 57 and the squeegee 56 is then stopped by a stopper 58. As a result of the horizontal movement of a printing head 59, paste 60 such as cream solder is printed on the substrate 51. Upon completion of the printing, the lower cylinder 53 moves the stage 52 downward and the head cylinder 57 moves the squeegee 56 upward.
In the first conventional screen printing apparatus, the lower limit of the squeegee 56 is determined by the stopper 58. Therefore, in order to select the most suitable printing condition, it is necessary for an operator to adjust the position of the stopper 58 by visually observing the surface state of the screen plate 55 and the printing state of the substrate 51 after printing is completed. Accordingly, printing state is varied depending on an operator, a substrate, or the period of time in which the paste 60 is exposed to air.
In addition, with the recent miniaturization of electronic appliances, the substrate on which electronic components are mounted is becoming smaller and finer. In this trend, there are growing demands for the development of screen printing apparatuses capable of not only accomplishing a favorable printing performance, but also having a mechanism for replacing a substrate-positioning stage in a short period of time.
The construction of a second conventional screen printing apparatus is described below with reference to FIG. 4. A substrate 62 transported by a transporting link 61 is positioned on the upper surface of a stage 63. A negative pressure-sucking means is employed to position the substrate 62 on the stage 63. Simultaneously with the upward movement of the stage 63 made by a cylinder 64, a protecting plate 65 is pressed upward by rods 66. Paste disposed on a screen 67 is printed on the substrate 62 with the upper surface of the substrate 62 and that of the protecting plate 65 moved upward to the flat lower surface of the screen 67.
In the construction of the second conventional screen printing apparatus, it is necessary to manufacture the stages 63 which is equipped with the substrate-sucking means and accords to the outer dimension of each substrate 62. Therefore, the manufacturing cost of this apparatus is high.
In addition, a factory requiring the stages 63 of various sizes may have a trouble that the stage 63 is not ready for a day when printing operation starts. Moreover, while the stage 63 is being replaced, the operation of the screen printing apparatus has to be stopped for a long time.
The construction of a third conventional screen printing apparatus is described below with reference to FIG. 5. A substrate 71 is supported by support pins 70 inserted into holes 69 of a stage 68 and a cylinder 72 moves horizontally presses rollers 73 against the periphery of the substrate 71 so that the substrate 71 is positioned on the stage 68. Then, a cylinder 74 moves the substrate 71 so that the substrate 71 is positioned at a level in the vicinity of the lower surface of a screen 75 and then paste 76 placed on the screen 75 is printed on the substrate 71 with a squeegee 77 which moves horizontally sweeping the paste 76.
In the construction of the third conventional screen printing apparatus, the selected support pins 70 are inserted into the holes 69 of the stage 68 according to the outer dimension of the substrate 71. Therefore, it is unnecessary to manufacture the stages 68 of various sizes unlike the second conventional screen printing apparatus. The operation efficiency of the screen printing apparatus is favorable in this respect. But it takes much time and labor to insert the support pins 70 into the holes 69 of the stage 68 if the paste 76 is to be printed on both surfaces of the substrate 71. That is, it is necessary for an operator to selectively insert the support pins 70 into the holes 69 of the stage 68 in the screen printing apparatus so that the support pins 70 do not contact electronic components mounted on either the front surface of the substrate 71 or the back surface thereof.